Method and apparatus for implementing automatic-calibration of TDR probing system

ABSTRACT

A method and apparatus are provided for implementing automatic-calibration of a Time Domain Reflectometer (TDR) probing apparatus. A calibration procedure is performed automatically each time a TDR probe is moved from a device under test (DUT). A current calibration TDR waveform is obtained and compared with a reference calibration TDR waveform, checking for deviations between the current and reference measurements. If a deviation is detected, then the user is notified and calibration is failed.

FIELD OF THE INVENTION

The present invention relates generally to the data processing field,and more particularly, relates to a method and apparatus forautomatically implementing calibration of a Time Domain Reflectometer(TDR) probing apparatus used for testing a printed circuit board.

DESCRIPTION OF THE RELATED ART

As used in the present specification and claims, the term printedcircuit board or PCB means a substrate or multiple layers (multi-layer)of substrates used to electrically attach electrical components andshould be understood to generally include circuit cards, printed circuitcards, printed wiring cards, and printed wiring boards.

Characteristic impedance is most commonly measured using time-domaintechniques, typically time-domain reflectometry (TDR). Currentlyproblems result from some manufacturers using uncalibrated TDR testequipment for screening raw card impedance. TDR test equipment thatfails calibration tests may be identified weeks or months after aninitial calibration failure. Either calibration procedures are notperformed or are not performed often enough to appropriately screenparts. This failure can result in significant expense to a manufacturerand its customers.

Significant problems are introduced by lack of adequate equipmentcalibration when performing impedance measurements and posting theresulting data. For example, rigid card and flexible printed circuitvendors are required to deliver product that meets predefinedcharacteristic impedance within a given tolerance. However, the data isonly valid when proper calibration schedules are maintained for the TDRtest equipment.

A need exists for an effective mechanism for improving the reliabilityof impedance data captured for printed circuit boards.

SUMMARY OF THE INVENTION

Principal aspect of the present invention is to provide a method andapparatus for automatically implementing calibration of a Time DomainReflectometer (TDR) probing apparatus. Other important aspects of thepresent invention are to provide such method and apparatus forautomatically implementing calibration of a Time Domain Reflectometer(TDR) probing apparatus substantially without negative effect and thatovercome many of the disadvantages of prior art arrangements.

In brief, a method and apparatus are provided for implementingautomatic-calibration of a Time Domain Reflectometer (TDR) probingapparatus. A calibration procedure is performed automatically each timea TDR probe is moved from a device under test (DUT). A currentcalibration TDR waveform is obtained and compared with a referencecalibration TDR waveform, checking for deviations between the currentand reference measurements. If a deviation is detected, then the user isnotified and calibration is failed.

In accordance with features of the invention, both the currentcalibration TDR waveform and the reference calibration TDR waveform areobtained using a known impedance standard contained within the TDRprobe.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above and other objects andadvantages may best be understood from the following detaileddescription of the preferred embodiments of the invention illustrated inthe drawings, wherein:

FIG. 1 is a block diagram representation illustrating an exemplarycomputer test system for implementing automatic-calibration of a TimeDomain Reflectometer (TDR) probing apparatus in accordance with thepreferred embodiment;

FIG. 2 is a more detailed schematic and block diagram representationillustrating exemplary apparatus of the computer test system of FIG. 1for implementing automatic-calibration of a Time Domain Reflectometer(TDR) probing apparatus in accordance with the preferred embodiment;

FIG. 3 is a flow chart illustrating exemplary steps of methods forimplementing automatic-calibration in accordance with the preferredembodiment;

FIGS. 4A, and 4B are a perspective view and a top view includinginterior details illustrating an exemplary two-way switch for use in thecomputer test system of FIG. 1 for implementing automatic-calibration ofa Time Domain Reflectometer (TDR) probing apparatus in accordance withthe preferred embodiment; and

FIG. 5 is a block diagram illustrating a computer program product inaccordance with the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with features of the invention, an enhanced TDR probe isprovided and used to implement an automatic-calibration method of theinvention. The enhanced TDR probe is used with a computer test systemfor implementing automatic-calibration of a Time Domain Reflectometer(TDR) probing apparatus. The enhanced TDR probe can be sold to acustomer as an upgrade and marketed as a productivity and yield/dataquality enhancement tool.

In accordance with features of the invention, the method of theinvention automatically checks the calibration of the probe and testequipment each time the user completes a normal measurement. Thisguarantees that the test setup is always calibrated and resultingmeasurement data is not subject to calibration error. Theautomatically-calibrated TDR probing apparatus is used to screen PCBsand printed flexible circuits for required characteristic impedanceprior to shipping product, and advantageously can ensure that onlyproduct within an acceptable characteristic impedance Zo specificationis shipped. Shipment errors due to calibration error are reduced sincethe operator is flagged the instant the probe is detected to be out ofcalibration, rather than the next morning or next week or whenever thecalibration procedure is performed again. Even when normal proceduresmay require daily calibration, these procedures are sometimes notperformed. Moreover, if a test setup drifts out of calibration withinthe specified timeframe, such as daily and the like, hundreds ofindividual product could be shipped out of specification within thistimeframe.

Referring now to the drawings, in FIGS. 1 and 2 there is shown acomputer test system generally designated by the reference character 100for carrying out methods for implementing automatic-calibration of aTime Domain Reflectometer (TDR) probing apparatus in accordance with thepreferred embodiment. Computer test system 100 includes a computer 102coupled to a data acquisition unit (DAU) 104. A Time DomainReflectometer (TDR) probe 106 in accordance with the preferredembodiment, a wrist strap 108 worn by a user of test system 100, and afoot petal 110 are coupled to the DAU 104. A normal measurement processis initiated via the small foot-pedal 110 actuated by the operator.

In accordance with features of the invention, user intervention is notrequired in order to perform a calibration procedure. Rather, acalibration procedure is handled automatically each time the user liftsthe TDR probe 106 from a device under test (DUT) 112. A normal orconventional calibration technique, such as a calibration procedureusing airlines or other known impedance comparisons, is handledautomatically to ensure that the recorded test data is accurate.

Computer test system 100 includes a mouse 114 and a printer 116 coupledto the computer 102. Data acquisition unit (DAU) 104 can be implemented,for example with CITS800 TDR test system manufactured and sold by PolarInstruments Inc., San Mateo, Calif. The TDR probe 106 can be implementedwith an upgraded and enhanced version of a TDR probe manufactured andsold by Polar Instruments Inc. TDR probe 106 of the preferred embodimentis illustrated and further described in more detail with respect to FIG.2.

While many printed circuit vendors utilize test equipment manufacturedby Polar Instruments Inc., and the automatic-calibration methods of thepresent invention improves the reliability of impedance data capturedusing Polar TDR equipment; it should be understood that the presentinvention can be applied to various other hand held TDR probes and TDRprobing systems.

As shown in FIG. 2, computer system 100 includes a main processor 128 orcentral processor unit (CPU) 128 and an operating system 130, acalibration control program 132 of the preferred embodiment, calibrationtest data 134, and a user interface 136 resident in a memory 138.

Various commercially available computers can be used for computer system100, for example, with computer 102 implemented by an IBM personalcomputer. CPU 128 is suitably programmed by the calibration controlprogram 132 for implementing automatic-calibration of a Time DomainReflectometer (TDR) probing apparatus in accordance with the preferredembodiment.

Computer system 100 is shown in simplified form sufficient forunderstanding the present invention. The illustrated computer system 100is not intended to imply architectural or functional limitations. Thepresent invention can be used with various hardware implementations andsystems and various other internal hardware devices.

FIG. 2 illustrates the TDR probe 106 receiving a TDR signal from the DAU104, for example, via a SubMiniature version A (SMA) connector 140. TDRprobe 106 includes a reference calibration impedance trace 142 carriedby a printed circuit board (PCB) 143 enclosed within a cosmetic plasticcase or housing 144 with a signal and ground pair probe tip generallydesignated by 146. Signal and ground pair probe tip 146 includes a pairof spring-load members 148 providing spring-loaded DUT contact points.TDR probe 106 includes a known calibration standard that is customizableper customer requirements and provided by the PCB trace 142 that is usedto calibrate the system.

While the known calibration standard is shown as a tightly controlledPCB reference calibration impedance trace 142 having a specificcharacteristic impedance, it should be understood that the knowncalibration standard could also be implemented by, for example, an airline, in accordance with the invention.

TDR probe 106 optionally includes an optical sensor 150, a lightemitting diode (LED) 151, and a single-pole double-throw (SPDT) relay152 that are used in conjunction with control logic circuit 154 toswitch the probe connection between a normal measurement trace pathincluding the probe tips 146 and a known-impedance calibration pathincluding the reference calibration impedance trace 142. An electronicpetal 156 is coupled between computer 102 and a signal combiningfunction 158 that is also coupled to the manually operated foot petal110. A USB port 160 of computer 102 and a USB port 162 of the electronicpetal 156 are used to couple a petal control signal applied by computer102 to the electronic petal 156 indicated at a line labeled PETAL CTLSIGNAL. A petal pressed signal indicated at a line labeled PETAL PRESSEDSIGNAL is applied by the signal combining function 158 to the DAU 104that provides the TDR signal to TDR probe 106. A data acquisition signalindicated at a line labeled DA SIGNAL is applied by the DAU 104 tocomputer 102 via a respective serial port 164 of computer 102 and aserial port 166 of the DAU 104. A calibration signal indicated at a linelabeled CAL SIGNAL is applied by the TDR probe 106 to computer 102 via arespective USB port 170 of computer 102 and a USB port 172 of the TDRprobe 106.

Referring now to FIG. 3, there are shown exemplary steps of methods forimplementing automatic calibration in accordance with the preferredembodiment staring at a block 300. A current calibration TDR waveform isobtained as indicated in a block 302. A stored reference calibration TDRwaveform, such as an immediately previous calibration TDR waveform iscompared with the current calibration TDR waveform checking fordeviations between the measurements as indicated in a block 304.

In accordance with features of the invention, computer 102 implementsthe automatic calibration method to confirm that a current calibrationTDR waveform compares with the previous calibration TDR waveform, bothusing the known impedance standard 142 and specifically looks fordeviations between measurements. If a deviation is detected as indicatedin a decision block 306, then the user is notified and calibration isfailed as indicated in a block 308. The user then can subsequentlyperform a complete diagnostic and calibration procedure per themanufacturers guidelines. Otherwise, if a deviation is not detected asindicated in a decision block 306, then the current calibration TDRwaveform is stored as indicated in a block 310. The automaticcalibration process ends as indicated in a block 312.

The calibration method of the invention ensures that the impedancemeasurement test system 100 maintains expected impedance and launchvalues from one measurement cycle/DUT to the next. This calibrationprocess is repeated automatically, between user measurements, withoutrequiring user intervention.

Computer 102 controlling the calibration process receives a calibrationsignal that indicates when the operator has lifted the probe 106 off ofthe DUT 112 to initiate the auto-calibration procedure. A preferredimplementation to provide the calibration signal is to use aprobe-mounted sensor, such as defined by optical sensor 150 and LED 151,to detect when the spring-loaded pins 148 are fully extended whenremoved from the DUT 112. The spring-loaded pins 148 are retracted whenthe probe tip 146 is pressed onto the DUT 112.

Simple control logic 154 on the PCB 143 inside the probe housing 144communicates to the computer 102 controlling the TDR test system 100 viathe USB or other standard port connections 170, 172. Test system 100then initiates the auto-calibration procedure using probe-mountedcontrol logic and the impedance controlled relay 152 to redirect the TDRsignal from the DUT, to the internal calibration standard 142. Oncecalibration has completed, for example, typically less than 1-2 seconds,the relay 152 resumes continuity of the transmission line to the DUT112.

Referring to FIGS. 4A, and 4B, there is shown an exemplary mechanicalcontrol switch generally designated by the reference character 400.Mechanical control switch 400 is implemented with the spring loadedprobe tips such that the switch between normal and calibrationmeasurement paths is made when the probe tip 146 is lifted from the DUT112.

Mechanical control switch 400 includes an actuator member 402 positionedfor sliding movement within a slot 404 of a switch housing 406. Actuatormember 402 is positioned within the TDR probe 106 for cooperativeengagement with the pair of spring-load members 148 for providingcontrolled switch movement.

As shown in FIG. 4B, the actuator member 402 includes an electricallyconductive member 410 illustrated in dotted line for providingcontrolled switch movement between respective pairs of a plurality ofelectrical contacts 412, 414, 416. A respective electrically insulativematerial 418 is provided between the electrical contacts 412 and 414 andelectrical contacts 414 and 416. The actuator member portion 410 ismoved corresponding to movement of the spring-load members 148 to switchthe probe connection between the normal measurement trace path includingthe probe tips 146 and the known-impedance calibration path includingthe reference calibration impedance trace 142.

Referring now to FIG. 5, an article of manufacture or a computer programproduct 500 of the invention is illustrated. The computer programproduct 500 includes a recording medium 502, such as, a floppy disk, ahigh capacity read only memory in the form of an optically read compactdisk or CD-ROM, a tape, a transmission type media such as a digital oranalog communications link, or a similar computer program product.Recording medium 502 stores program means 504, 506, 508, 510 on themedium 502 for carrying out the methods for implementingautomatic-calibration in accordance with the preferred embodiment in thesystem 100 of FIG. 1.

A sequence of program instructions or a logical assembly of one or moreinterrelated modules defined by the recorded program means 504, 506,508, 510, direct the computer test system 100 for implementingautomatic-calibration of the preferred embodiment.

While the present invention has been described with reference to thedetails of the embodiments of the invention shown in the drawing, thesedetails are not intended to limit the scope of the invention as claimedin the appended claims.

1. (canceled)
 2. A method for implementing automatic-calibration asrecited in claim 7 includes providing a reference calibration impedancewith the TDR probe.
 3. A method for implementing automatic-calibrationas recited in claim 2 wherein obtaining and comparing said currentcalibration TDR waveform with said reference calibration TDR waveformincludes using said reference calibration impedance for obtaining saidcurrent calibration TDR waveform.
 4. A method for implementingautomatic-calibration as recited in claim 3 includes using saidreference calibration impedance for obtaining said reference calibrationTDR waveform.
 5. A method for implementing automatic-calibration asrecited in claim 2 includes forming a printed circuit board trace fordefining said reference calibration impedance.
 6. (canceled)
 7. A methodfor implementing automatic-calibration of a Time Domain Reflectometer(TDR) probing apparatus comprising the steps of: performing acalibration procedure automatically each time a TDR probe is moved froma device under test (DUT); obtaining and comparing a current calibrationTDR waveform with a reference calibration TDR waveform includingswitching from a normal measurement path including a TDR probe tip to acalibration reference path including a reference calibration impedancetrace; providing an optical sensor for detecting the TDR probe beingmoved from the device under test (DUT); and a relay for switchingbetween said normal measurement path and said calibration referencepath; checking for deviations between the current calibration TDRwaveform and the reference calibration TDR waveform; and responsive todetecting a deviation, notifying a user of a failed calibration.
 8. Amethod for implementing automatic-calibration as recited in claim 7includes providing a mechanical control switch for switching betweensaid normal measurement path and said calibration reference path.
 9. Acomputer program product for implementing automatic-calibration of aTime Domain Reflectometer (TDR) probing computer test system, saidcomputer program product including instructions stored on a computerrecording medium and wherein said instructions, when executed by acomputer, cause the computer to perform the steps of: performing acalibration procedure automatically each time a TDR probe is moved froma device under test (DUT); obtaining and comparing a current calibrationTDR waveform with a reference calibration TDR waveform includingswitching from a normal measurement path including a TDR probe tip to acalibration reference path including a reference calibration impedancetrace; providing an optical sensor for detecting the TDR probe beingmoved from the device under test (DUT); and a relay for switchingbetween said normal measurement path and said calibration referencepath; checking for deviations between the current calibration TDRwaveform and the reference calibration TDR waveform; and responsive todetecting a deviation, notifying a user of a failed calibration.
 10. Acomputer program product for implementing automatic-calibration asrecited in claim 9 wherein obtaining and comparing said currentcalibration TDR waveform with said reference calibration TDR waveformincludes using a reference calibration impedance provided with the TDRprobe for obtaining said current calibration TDR waveform.
 11. Acomputer program product for implementing automatic-calibration asrecited in claim 10 includes using said reference calibration impedanceprovided with the TDR probe for obtaining said reference calibration TDRwaveform.
 12. A computer program product for implementingautomatic-calibration as recited in claim 9 includes responsive todetecting no deviations, storing said current calibration TDR waveform.13. (canceled)
 14. (canceled)
 15. (canceled)
 16. Apparatus forimplementing automatic-calibration of a Time Domain Reflectometer (TDR)probing apparatus comprising: a computer; a TDR probe coupled to saidcomputer; said TDR probe including a reference calibration impedance anda control logic circuit for coupling a calibration signal to saidcomputer when said TDR probe is moved from a device under test (DUT);said reference calibration impedance including a printed circuit board(PCB) trace for defining said reference calibration impedance; said TDRprobe including a normal measurement path including a TDR probe tip anda calibration reference path including said reference calibrationimpedance PCB trace; said TDR probe including an optical sensor fordetecting the TDR probe being moved from the device under test (DUT);and a relay for switching between said normal measurement path and saidcalibration reference path; said computer for performing a calibrationprocedure automatically each time said TDR probe is moved from thedevice under test (DUT) including the steps of: obtaining and comparinga current calibration TDR waveform with a reference calibration TDRwaveform; checking for deviations between the current calibration TDRwaveform and the reference calibration TDR waveform; and responsive todetecting a deviation, notifying a user of a failed calibration. 17.Apparatus for implementing automatic-calibration as recited in claim 16wherein said TDR probe includes a mechanical control switch forswitching between said normal measurement path and said calibrationreference path.